Generally, an electrophotographic image forming apparatus forms an image by exposing a photosensitive body (image carrier) such as a photosensitive drum or photosensitive belt with a laser beam or the like to form an electrostatic latent image corresponding to an image signal, developing the electrostatic latent image, and transferring it onto a printing medium or the like. At this time, the laser beam is scanned sequentially one-dimensionally in the main scanning direction, for example, from left to right. At the same time, the laser beam is scanned in a direction almost perpendicular to the main scanning direction, for example, from top to bottom. On the image carrier, many straight lines (to be referred to as scanning lines) in the main scanning direction are formed parallelly at a predetermined interval (to be referred to as a reference scanning line interval) in the sub-scanning direction.
An image formed by an image forming apparatus of this type generates a horizontal stripe (to be referred to as banding) upon variations of the image density owing to various causes, greatly degrading the image quality.
For example, an image carrier speed error causes a scanning line interval error, generating banding. More specifically, even if the entire photosensitive body is to be exposed uniformly, when its rotational speed is high, the exposure amount decreases because the scanning line interval widens, and when the rotational speed is low, increases because the scanning line interval narrows. As a result, the density of the formed image varies.
A laser beam irradiation intensity error also generates banding. More specifically, even if the entire photosensitive body is to be uniformly exposed, the exposure amount decreases when the laser beam irradiation intensity is low, and increases when it is high. The density of the formed image varies as well.
As a method for correcting banding, there is proposed a method of detecting a scanning line position error caused by an image carrier speed error, calculating the interval from an immediately preceding scanning line based on the position error, and adjusting the exposure amount based on the interval (see, for example, Japanese Patent Laid-Open No. 2-131956).
In general, the image density of a formed image is affected by the density of neighboring scanning lines. Hence, banding correction by adjustment of the exposure amount requires position information of neighboring scanning lines in addition to the current scanning line. However, in the method disclosed in Japanese Patent Laid-Open No. 2-131956, scanning line interval information used in correction is only information of the interval between the current scanning line and the immediately preceding scanning line. Also, the correction rate or the like is not particularly calculated, and no satisfactory correction may be done.
Japanese Patent Laid-Open No. 5-002317 discloses an invention which reduces banding by detecting an image carrier speed error and correcting the exposure intensity of a laser beam to keep constant the exposure amount of the image carrier per unit area.
As a matter of course, the local density of an image is affected by dots which form the image. For example, when printing a so-called solid image by forming dots on the entire image surface, if the image carrier speed is lower than the reference, dots become dense and the image density increases, and if it is higher, the image density decreases. Controlling the exposure amount by the invention of Japanese Patent Laid-Open No. 5-002317 can prevent a change of the image density caused by an image carrier speed error, forming an image at an almost uniform density. However, a discrete dot or so-called isolated point is not affected by neighboring dots. To adjust the density of the isolated point to a desired value, the dot needs to be formed at the same exposure intensity regardless of the image carrier speed.
The concept of density correction will be explained with reference to the schematic view of FIG. 16. In FIG. 16, a shows a state in which the reference line interval is maintained and the dot interval is constant. In this case, the exposure intensity suffices to be constant between scanning lines. In FIG. 16, b shows a state in which the scanning line interval widens between lines N and N+2 and narrows between lines N+3 and N+5. In this case, the exposure intensity of line N+1 is increased and that of line N+4 is decreased. Accordingly, a change of the image density by an image carrier speed error can be prevented, forming an image at an almost uniform density.
However, color dots are not always formed on all lines aligned in the sub-scanning direction except for a solid image. In FIG. 16, c shows an example of forming white dots indicated by broken lines on lines N, N+2, N+3, and N+5, and forming color dots on lines N+1 and N+4. In this case, to form color dots (isolated points) on lines N+1 and N+4 at a desired density, the exposure intensity should not be changed even if the scanning line interval varies as in b of FIG. 16. That is, to form a given point of an image at a desired density, the exposure intensity needs be controlled in accordance with the image carrier speed error, and at the same time, a dot pattern around the point needs to be considered.
Japanese Patent Laid-Open No. 2-131956 discloses an invention in which a scanning line position error by an image carrier speed error is detected, the interval from an immediately preceding scanning line is calculated from the detected error, and the exposure amount is adjusted based on the calculated interval.
The density of a formed image is affected by the density of scanning lines. Thus, banding correction by adjustment of the exposure amount requires information of neighboring scanning lines in addition to that of the current scanning line. However, in the invention of Japanese Patent Laid-Open No. 2-131956, scanning line interval information used in correction is only information indicating the interval between the current scanning line and the immediately preceding scanning line. Banding cannot be corrected sufficiently.
If the position of a scanning line succeeding the current scanning line is predicted, information of neighboring scanning lines in addition to the current scanning line can be obtained, and the exposure amount can be adjusted using the information. However, if the predicted position of a scanning line or the exposure amount to be adjusted has an error, no banding can be corrected appropriately.